On August 25, the National Park Service will allow entrance to all 401 National Parks for free to celebrate the National Park Service's Birthday. That's a savings of up to $25 (at Grand Canyon National Park, the most expensive national park).

http://www.nps.gov/findapark/feefreeparks.htm

Of note, admission to most national parks is usually free; click here for a list of parks which offer free admission on this day only.